This invention relates generally to television tuning systems having automatic frequency control and specifically to television electronic tuning systems in which automatic frequency control is accomplished by microcomputer-controlled frequency synthesis.
Automatic frequency control (AFC) systems are, in general, well known in the art and typically involve heterodyning the transmitted signals picked up by the receiver's tuner with locally generated oscillator signals and producing intermediate frequency (IF) signals for processing. This frequency translation process is well known and is necessitated by a variety of factors including the inherent characteristics of the electronic components utilized. In television receivers the local oscillator frequency is adjusted by means of a control voltage to heterodyne with the picture carrier in producing an IF picture carrier of 45.75 MHz. Each television channel transmits at a different carrier frequency with the individual channels separated by a designated frequency increment of 6 MHz from channel to channel except for channels 5-6 (10 MHz), channels 6-7 (92 MHz, and channels 13-14 (260 MHz).
In processing the received television signals, a discriminator or detector having a tuning or center frequency of 45.75 MHz, corresponding to the picture IF, is supplied with the IF signal and develops an error voltage dependent upon the polarity of the difference between the received picture IF carrier and the center frequency of the discriminator. The error voltage is fed to appropriate circuitry in the tuner for controlling the frequency of the local oscillator based upon polarity and/or magnitude of this difference signal. Changes in the oscillator frequency in response to the error voltage are employed to bring the received picture IF carrier frequency in close coincidence with the center frequency of the detector. Tuning equilibrium is achieved when the error voltage produced and the system gain operate on the local oscillator to maintain tuning close to the detector center frequency at which point the receiver should be properly tuned, i.e., to within its specified frequency tolerance.
The proliferation of CATV, MATV and inexpensive television games has placed increasing demands on AFC systems. In some CATV or MATV installations, the channel oscillator frequencies are offset from the FCC-designated standard for broadcast transmitters. This frequency offset is intentionally introduced in these systems to reduce interference. The frequency offset in television video games is generally attributed to the wider tolerances in terms of nominal operating frequency, frequency stability, etc., of the oscillators utilized. To further complicate the situation the all-electronic television tuning systems currently being installed derive the channel number from the incoming signal, a preprogrammed memory or from the oscillator frequency. These all-electronic tuning systems do not incorporate the manually-operated fine tuning control which enabled the older tuning systems to receive the proper television signals even though the nominal oscillator frequency differed from its correct value. These difficulties have placed even more stringent tuning requirements on current AFC systems to acquire sometimes significantly offset video signals while simultaneously being able to precisely tune to those VHF and UHF channels which are operating at their designated frequency.
One approach taken in attempting to reconcile these competing objectives has involved the use of microcomputer-controlled tuning systems. These systems typically utilize a memory matrix for storing digital tuning data and updating stored tuning information periodically as dictated by tuning system control logic. One example of a digital computer-controlled approach to TV tuning is disclosed in U.S. Pat. No. 4,093,922. Involved therein is the use of a non-volatile ROM memory matrix for storing binary tune words which are updated either manually by external user control or by AFC tuning. The means of achieving digital AFC control is not disclosed in this patent wherein it is stated that the digital control system could be incorporated in the system by means of a couple of comparator windows and the addition of appropriate control logic to the system's algorithm. A typical AFC subroutine would involve the stepwise incrementing or decrementing of the tuning voltage over a particular tuning range corresponding to the selected channel. This approach leads to tuning inaccuracies because of its inherent incremental character wherein the channel being sought could be grossly mistuned by operating at a frequency located between two adjacent frequency steps and because of the difficulty in accurately measuring and adjusting to precise window and edge tuning information.
Another approach to improving television receiver tuning accuracy is disclosed in U.S. Pat. Nos. 4,041,535 and 4,025,953 wherein a frequency synthesis technique is utilized in tuning the television receiver to the desired channel. This approach involves the use to two programmable frequency dividers, the first connected between the output of a reference oscillator and one input to a phase comparator and the second connected between the output of a local oscillator and the other input to the phase comparator. The first programmable divider is controlled by the channel selector as the system tunes to the selected channel number. The second programmable divider receives inputs from logic circuitry which is responsive to predetermined relationships of signals from a picture carrier detector, a sound carrier, an AFT discriminator circuit, and the presence of vertical synchronization signal components. The phase comparator output is a tuning voltage used to control the tuning of the local oscillator. It is in this manner that the reference oscillator frequency supplied to the phase comparator is automatically changed to correct for received carrier signal frequency offset which may be in excess of the pull-in range of the AFT discriminator circuit. This approach, however, suffers from several performance limitations in that component ageing may change tuning system accuracy which this approach cannot compensate for and the RF environment in which the receiver actually operates may differ significantly from that which it was originally set up to operate in.
Another approach involving the microcomputer control of a television tuning system is disclosed in "A Microcomputer Controlled Frequency Synthesizer for TV", IEEE Transactions on Consumer Electronics, Volume CE-24, No. 2, May 1978, by Rzeszewski et al. The approach disclosed therein is very similar to that referred to in the previous paragraph with the addition of a microcomputer for controlling the phase lock loop (PLL) which is constantly correcting for drift in local oscillator phase insuring that the local oscillator is always precisely in phase with the fixed-frequency crystal oscillator. This provides for accurate input to the phase comparator which, in turn, controls the operation of the local oscillator. Automatic fine tuning is accomplished by changing the input to a programmable counter between the local oscillator and phase comparator. The step-wise operation of the programmable counter provides for frequency increment steps ranging from 28 KHz for channel 2 to 70 KHz for channel 13 for automatic off-set correction on VHF. Because the step-wise tuning operation ceases only upon detection of nominal carrier frequency cross-over, this approach can result in television receiver tuning errors of as much as approximately 28 KHz in the VHF band.
In accordance with the present invention, these tuning system limitations are substantially reduced by performing AFC functions under the control of a microcomputer in which a return function is executed during the stepwise tuning cycle to more accurately tune the receiver to the desired frequency. This approach provides the increased tuning flexibility not available in the conventional dedicated control circuit tuner. This flexibility is particularly desirable in an RF environment containing precisely tuned as well as grossly offset television channels.